Various pieces of information such as control and image data are transmitted between a host system implemented by a data processing unit and an input/output unit implemented by, for example, a printer apparatus in an information processing equipment such as a computer. The transmission of data between such a host system and an input/output unit is dictated by a communications protocol which consists of rules and formats governing the communication of data therebetween. In order that the host system and input/output unit operate on such a communications protocol, the protocol must be decipherable by both the host system and the input/output unit. In this instance, it is an ordinary practice that the protocol for the input/output unit be determined or selected to conform to that workable in the host system and, for this reason, a printer apparatus which is typical of the input/output unit is designed for exclusive use with the host system. If a protocol incompatible with that used in the host system is to be adopted for an input/output unit, the input/output unit is required to be equipped with any protocol converter to convert the protocol of the host system into the protocol used in the input/output unit.
A representative example of such a protocol converter has incorporated therein a program by means of which the control data output from the host system is to be converted into corresponding data operable in the input/output unit. The program conversion program, or emulation software program, is activated by a control data processor provided in the input/output unit. In an input/output unit using such a control data processor, it is also required that the control data once converted from the protocol of the host system be further converted into the form of internal parameters effective to control the various modes and phases of operation of the printer apparatus.
In a printer apparatus for use in an information processing equipment such as a computer is ordinarily provided a function with which prescribed alphanumerical letters and graphical patterns are to be printed when a particular one of the keys on a control panel is depressed by the operator of the apparatus.
When a test printing mode of operation is selected in a prior-art printer apparatus of this type and the printer apparatus is in operation on an emulation software program, printing operation is performed in similar manners as those which are followed when the printer apparatus operates on the protocol specific to the apparatus. However, when the printer apparatus is in operation on an emulation software program, it is sometimes desirable that information peculiar to the emulation software program be reproduced on the software program. Such a demand can not be satisfied in a printer control system which has been developed for use in a conventional printer apparatus having a test printing function.